River and reservoir basins are operated for various purposes including power production, irrigation, environmental needs, recreation, and water quality. In many basins, water managers must track not only the volume and flow of water throughout the basin, but also the water ownership and type of water.

Operating decisions in the basin are dependent on many factors, including a user’s available water, legal restrictions, physical constraints, and any exchange mechanism. In addition, many basins in the United States are governed by the doctrine of Prior Appropriation, which says that the right to use a water supply is based on first in time, first in right.

Because of these constraints, it is necessary for water managers to have a tool to simulate the operating decisions and their effect in the basin, including water type and water ownership. The requirements for such a simulation tool are as follows.

RiverWare meets these requirements through water accounting. Simply, water accounting is a layer added to a simulation model to track the ownership and type of the simulated water. The following box highlights how these requirements are met by the RiverWare water accounting functionality. These pieces are described in the following text as an introduction to water accounting and in more detail in this document.

In an accounting model, accounts, slots, and data are added to track why water is released, stored, or diverted. These additional components include water ownership, type, and purpose as water moves through the basin. This network is separate from the simulation network, but there are methods that allocate physical water to the accounting network.

Accounts are linked to one another on both the same object and on different objects. These links indicate the possible transfers of water in the system. By defining a link, the user indicates that at some timestep, water could move between the two linked accounts. Legal accounts, including Storage, Diversion, and Instream Flow, are used to model a legal water right. Passthrough accounts are used to track the transfer of water through basin features. With both legal and non-legal accounts, the user can look at any object in the basin and determine the type and ownership of all of the water in that object.

Rules can access the account solution and can also control and set the releases from the accounts. User inputs can also drive the account solution. Using rules and other accounting utilities, the user can simulate typical accounting functionality, including accrual, exchanges, carryover, and allocation. In addition, a special predefined rule function and set of methods serves as a water rights allocation solver to allocate water to legal accounts based on prioritized water rights.

Water accounting can be used for a number of purposes. These include after-the-fact accounting, daily operations, and long term planning as described as follows:

An after-the-fact accounting model is used on a timestep by timestep basis to track the type and ownership of water that was released the preceding timestep. For example, a water district might release water for a number of specific purposes on a given day. The following day the gaged data is imported into the model and final storages, evaporations, and losses can be calculated. The water district is able to then track how those actual releases and losses in the system should be charged to the various uses. This is considered an after-the-fact accounting model because the physical volumes, losses, and flows are known; the accounting model must calculate how those operations should be charged to the users in the basin. Often this is an exercise in book-keeping.

A short-term operations model uses information from the previous days and forecasts to determine the operations for future timesteps. If the operations depend on the amount of water available to specific users in the basin, then an accounting system is necessary to track how those releases will be made. In practice, rules set releases after looking at the balance on a certain account. For example, if a user requests a release, but that user has no available water in the accounting system, no physical release can be made.

Long term planning models are used to determine the effect operations have on a longer time scale. If the operations depend on water ownership or type in the basin, then an accounting model can be used to track those pieces. The model must look at the state of the system including water ownership and type at each timestep and determine the operations to perform.

Often, water managers may use each of the three types of accounting models in a basin for water management. An after-the-fact accounting model is used each timestep to account for the previous timestep. The results from the after-the-fact model are fed into a short-term operations model to determine upcoming operations. Finally, a long term planning model is used for impact analysis, yield determination, and other planning related purposes. The structure; that is, the physical layout of objects and accounts in the model, are the same in each model, only the operating policy rules and data are different.

Let us develop some definitions to better discuss the accounting system. Physical Water is modeled using the simulation objects and does not have a unique classification. For example, the outflow from a reservoir is considered physical water. It may be released for a number of downstream purposes, but the total outflow is the value on the Outflow slot.

To contrast physical water, in the accounting system there is Paper Water (sometimes called colored water). This is the water that has a specific owner or type that must be tracked.